clang/test/Driver/darwin-sdkroot.c: Suppress this on msys bash, to introduce the feature "shell-preserves-root".

MSYS transforms '/' to 'X:/mingwroot/'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172598 91177308-0d34-0410-b5e6-96231b3b80d8